I had a similar experience with my 5KTQW not that long ago, only I was
not lucky enough to have to have it happen in my drive. On mine, there
still was about 1+" of stub left on the rad. Enough to reattach the
hose, fill the rad and drive the rest of the way home. might have worked
for a week or two, but I did not push it.
My research with some local mechanics seem to say that once the nipple
breaks off, the fix is not worth it (unless you are very broke). You
can try epoxying the part back together with a sleeve of some sort. But
usually by the time the plastic breaks, it is worn out, and likely to
break again elsewhere soon. On mine the plastic at the break was about
twice as thick as on a new one, from the plastic deteriorating, which
goes along with this theory.
I ended up going with the Audi rad as a replacement. The original one
lasted 170K miles, and that seemed good enough. And there were threads
a while ago of problems with some of the aftermarket ones (sorry forgot
the brand name).
HTH